1. Field of the Invention
The present invention relates to a memory cell capacitor and method for forming said memory cell capacitor. More particularly, the present invention relates to a method of forming memory cell capacitors by efficiently utilizing the area over the surface of a semiconductor substrate.
2. State of the Art
A widely-utilized DRAM (Dynamic Random Access Memory) manufacturing process utilizes CMOS (Complementary Metal Oxide Semiconductor) technology to produce DRAM circuits which comprise an array of unit memory cells, each including one capacitor and one transistor, such as a field effect transistor. In the most common circuit designs, one side of the transistor is connected to one side of the capacitor, the other side of the transistor and the transistor gate are connected to external circuit lines called the bit line and the word line, and the other side of the capacitor is connected to a reference voltage that is typically one-half the internal circuit voltage. In such memory cells, an electrical signal charge is stored in a storage node of the capacitor connected to the transistor that charges and discharges the circuit lines of the capacitor.
Higher performance, lower cost, increased miniaturization of components, and greater packaging density of integrated circuits are ongoing goals of the computer industry. The advantages of increased miniaturization of components include: reduced-bulk electronic equipment, improved reliability by reducing the number of solder or plug connections, lower assembly and packaging costs, and improved circuit performance. In pursuit of increased miniaturization, DRAM chips have been continually redesigned to achieve ever-higher degrees of integration. However, as the dimensions of the DRAM chips are reduced, the occupation area of each unit memory cell of the DRAM chips must be reduced. This reduction in occupied area necessarily results in a reduction of the dimensions of the capacitor, which, in turn, makes it difficult to ensure required storage capacitance for transmitting a desired signal without malfunction. However, the ability to densely pack the unit memory cells, while maintaining required capacitance levels, is a crucial requirement of semiconductor manufacturing if future generations of DRAM chips are to be successfully manufactured. This drive to produce smaller DRAM circuits has given rise to a great deal of capacitor development.
In order to minimize such a decrease in storage capacitance caused by the reduced occupied area of the capacitor, the capacitor should have a relatively large surface area within the limited region defined on a semiconductor substrate. However, for reasons of available capacitance, reliability, and ease of fabrication, most capacitors are stacked capacitors in which the capacitor covers nearly the entire area of a cell and in which vertical portions of the capacitor contribute significantly to the total charge storage capacity. In such designs, the side of the capacitor connected to the transistor is generally called the xe2x80x9cstorage nodexe2x80x9d or xe2x80x9cstorage polyxe2x80x9d (since the material out of which it is formed is doped polysilicon) while the polysilicon layer defining the side of the capacitor connected to the reference voltage, mentioned above, is called the xe2x80x9ccell polyxe2x80x9d.
U.S. Pat. No. 5,292,677 issued Mar. 8, 1994 to Dennison and U.S. Pat. No. 5,459,094 issued Oct. 17, 1995 to Jun each teach methods for fabricating capacitors for memory cells. However, as with other known fabrication methods, these methods require numerous complex steps in forming the capacitors and do not maximize the size of the capacitor by efficient use of the space above the semiconductor substrate.
Therefore, it would be advantageous to develop a technique for forming a high surface area capacitor and a memory cell employing same, while using inexpensive, commercially available, widely practiced semiconductor device fabrication techniques and apparatus without requiring complex processing steps.
The present invention is a novel masking and etching technique for the formation of a memory cell capacitor and a memory cell by forming containment recesses which efficiently utilize the space above the semiconductor substrate. The capacitors are made by using thin spacers to pattern barrier material separation walls between the capacitors. This allows the capacitors to utilize the maximum amount of the area on the surface of the chip by minimizing the amounts of the barrier material present. The efficient utilization of the space above the semiconductor substrate increases the surface area of the storage poly node. The increase in the storage poly node surface area results in increased memory cell capacitance without complex processing steps.
The method of the present invention occurs after formation of an intermediate structure comprising transistor gates on a silicon substrate which has been oxidized to form thick field oxide areas and which has been exposed to implantation processes to form drain and source regions. The intermediate structure further comprises at least one barrier layer which covers the transistor gates and the silicon substrate.
The method of the present invention comprises patterning a first resist on the barrier layer. The pattern is a predetermined pattern which ultimately forms a specifically shaped capacitor. After the first resist is patterned, the barrier layer is lightly etched to a predetermined depth. The first resist is then stripped and a shield layer is deposited over the etched surface of the barrier layer. A second resist is patterned on the shield layer. The shield layer is then etched with a selective etchant to etch the shield layer material such that a portion of the shield layer under the second resist and a portion in corners of the etched barrier layer (hereinafter, xe2x80x9cthe thin spacersxe2x80x9d) remain. Thus, the depth of the light etch in the barrier layer must be sufficient to achieve a desired height of the thin spacers. Selective etching, as referred to herein, relates to using etchants which etch only a particular material while being substantially inert to other materials.
The barrier layer is then etched with an etchant selective to the buffer layer in order to expose a portion of the transistor gates, a portion of the active areas, and a portion of the field oxide areas. This etching forms bitline areas under the second resist and barrier material separation walls under the thin spacers. The second resist is removed. A storage poly layer for the lower cell plate of the capacitor is deposited over the exposed transistor gates, the exposed active areas, the exposed field oxide areas, the bitline areas and barrier material separation walls. A support material is applied over the lower cell plate. The structure is then planarized to remove the silicon nitride layer portions. This planarization also separates the storage poly layer into individual capacitor areas.
A dielectric layer is deposited over the storage poly layer and the exposed portion of bitline areas and barrier material separation walls. A cell poly layer is then deposited over the dielectric layer. A resist layer is patterned on the cell poly layer, and the cell poly layer and dielectric layer are etched to expose a portion of each bitline area over an area where a bitline will be formed. Subsequent steps known in the art are used to form the bitline and complete the memory circuit.